This week's news about a case of mad cow disease in Canada, the eighth confirmed incident in North America, has led to calls for a ban on imports of Canadian cattle and a permanent ban on sick or injured cattle ("downer" cows) being allowed to enter the human food chain.

It also raises questions about the US Department of Agriculture's testing and tracking procedures, as well as its controversial prohibition on private testing. Creekstone Farms of Campbellsburg, Ky., wants to privately test all its cattle so it can market its beef in countries that currently refuse to accept US beef. Having been blocked from doing so by the USDA, Creekstone Farms filed suit over the issue last month. The USDA argues that under current law it has the sole authority to issue the test kits for mad cow disease.

Meanwhile, some farmers worry that the USDA's National Animal Identification System, set to begin next year and become fully operational by 2009, will hurt families with small farms, homesteaders, and even pet owners.

"Every horse, cow, pig, goat, sheep, alpaca, et cetera will have to be tagged with electronic tags at the farmer's expense, and the farmer must register his premise ID for a fee yet again for the privilege to own such animals, even if it is just one," says Sharon Davis, who describes herself as a "small time farmer" in Pine Mountain, Ga.

All of this points up the potential conflict between the USDA's two basic mandates: Promoting US agricultural products domestically and abroad even as it is ensuring the safety of those products, which could sometimes require revealing bad news to consumers.

Mad cow disease, known as bovine spongiform encephalopathy (BSE), is believed to be carried by animal feed made from the remains of slaughtered cattle. Such feed has been banned in the US and Canada since 1997, but cases of BSE have continued to appear. The human form of the disease contracted from consuming beef products of infected cattle is the main concern. Some 150 people in Europe died as a result of it in the 1980s and 1990s.

This week's case, discovered in the Fraser Valley of British Columbia, involves a cow born after the feed rules were set nine years ago. Canadian authorities assure consumers that the cow did not enter the human food chain, and US officials have joined the investigation.

"It is important to note that Canada's monitoring system identified this animal as one that should be removed from the food and feed supply chain," said US Agriculture Secretary Mike Johanns.

Critics disagree. "This shows that the feed restrictions in place in Canada, and similar restrictions in the United States, are simply not adequate to control the spread of this disease," says Michael Hansen, of Consumers Union, an independent testing and information nonprofit. "There is no fire wall."

USDA officials say increased inspections in recent years, plus the plan to identify and track farm animals for human consumption, means a very safe system covering upwards of 100 million cattle in the US.

"I want to emphasize that human and animal health in the United States are protected by a system of interlocking safeguards, and that we remain very confident in the safety of US beef," USDA Chief Veterinary Officer John Clifford said last month when the latest case of mad cow in the US (the third) was discovered on a farm in Alabama.

An animal ID system will help health officials identify where a diseased animal was born and shorten the time it takes to trace the animal's history to identify other potentially exposed animals, Secretary Johanns said in a press conference earlier this month.

Since the first US case of mad cow was discovered in 2003, the number of cattle tested for the disease has increased substantially. Still, the rate of inspection remains far lower than in Europe or Japan - about 1 percent of the 35 million cows slaughtered annually in the United States.

Preventing contaminated feed and the slaughter for consumption of "downer" cattle unable to stand or walk are key to detecting and avoiding the disease, says the USDA. Creekstone Farms is challenging the USDA's exclusive authority to allow tests.

"We're not in any way saying that US beef isn't safe. We believe it's the safest beef supply in the world, but that's not the issue," Creekstone Farms founder and CEO John Stewart said in filing his company's lawsuit last month.

But "if BSE testing is an additional attribute that our customers want, free enterprise should allow us to provide this additional element," he added.